We will start with electromagnets and see how they are measured, because in some aspects, it is easier to first understand electromagnets.

Consider the two electromagnets shown above. The one on the right is a simple insulated copper wire coil wound around a piece of iron, such as a nail.

The one on the left is the same, but the iron core has been bent into the shape of the letter "C", much like a horseshoe magnet.

The left image makes it easier for us to see the so-called magnetic circuit coil creating a magnet with North and South poles, and the magnetic force moves in a circuit from north to south, passing through the air gap to reach there.

In this regard, the magnetic circuit is similar to the circuit. Current always flows from one electrode to another, just like current flows from one end of a battery to the other.

Magnetic flux is unwilling to propagate in the air. It is much easier to pass through iron. We say that air has a high reluctance and iron has a low magnetic resistance. This is similar to the resistance in a circuit.

In circuits, we have Ohm's law, which states that voltage is equal to current multiplied by resistance. Similarly, the magnetomotive force is equal to the magnetic flux multiplied by the magnetic resistance. Therefore, the magnetic electromotive force is similar to voltage, and the magnetic flux is similar to current.

The magnetic electromotive force is generated by the coil. Its unit is the number of ampere turns, which is the current in the coil, measured in amperes, multiplied by the number of turns of the wire in the coil.

If we know the length, area, and permeability of iron and air in the iron core. Just like calculating the resistance of a wire, the larger the cross-sectional area, the smaller the magnetic resistance, and the longer the length, the greater the magnetic resistance.